Naphtha cracking produces various hydrocarbon compounds such as ethylene, propylene and butene, and the production percentages of these olefins are inevitably fixed. The proportions in which these olefins are produced are not always in agreement with the olefin demands. It is therefore necessary that the olefins from the naphtha cracking are converted to meet the demands. Metathesis reaction is used for the conversion of olefins. In the metathesis reaction, olefins of the same or different kinds are reacted with each other to yield olefins having different structures. The reaction is of high importance because it is capable of converting the olefins from the naphtha cracking in response to changes in olefin demands.
In 1931, it was found that the olefin metathesis reaction proceeded at a high temperature of 725° C. without catalysts. However, it was not until the development of a catalyst having an oxide of metal such as molybdenum, tungsten or rhenium supported on a large-surface area carrier that the industrial value of the metathesis reaction was acknowledged. The first metathesis reaction with such catalysts was developed by Phillips Petroleum Company in 1964 wherein ethylene and 2-butene were synthesized from propylene under catalysis of molybdenum oxide supported on γ-alumina.
The metathesis reaction is reversible, and thus there exists an equilibrium composition. For example, in the metathesis reaction of ethylene and 2-butene into propylene, the equilibrium shifts such that more propylene is produced at lower temperatures. Studies have been then carried out to lower the reaction temperature by improving catalysts. Phillips Petroleum Company developed a method wherein a catalyst comprising tungsten oxide supported on silica, and co-catalyst magnesium oxide are used in combination. This method has been established as a propylene production process by Lummus Global, Inc.
In particular, U.S. Pat. No. 4,575,575 (Patent Document 1) and Journal of Molecular Catalysis, Vol. 28, p. 117 (1985) (Non-Patent Document 1) report that when a metathesis reaction between ethylene and 2-butene is carried out at 330° C. in the presence of silica-supported tungsten oxide as a catalyst, the conversion of butene is only 31%, while when magnesium oxide is used in combination as a co-catalyst, the conversion is enhanced to 67%.
Moreover, U.S. Pat. No. 4,754,098 (Patent Document 2) reports that in the same metathesis reaction at 330° C., the use of a co-catalyst in which magnesium oxide is supported on γ-alumina increases the conversion of butene to 75%. It is also reported in U.S. Pat. No. 4,684,760 (Patent Document 3) that when a co-catalyst comprising magnesium oxide and lithium hydroxide supported on γ-alumina is used, the butene conversion is maintained at 74% even at a lower temperature of 270° C.
However, the reaction temperatures described in the above documents (for example, 270° C. in Patent Document 3) are still high considering the practical industrial process, and there are needed facilities such as heating furnaces in order to achieve such reaction temperatures, for example 270° C. The reaction temperature in the metathesis reaction should be therefore lowered to a level that is more simply achievable by steam heating, for example, approximately 200° C.
To solve this problem, the present applicant has already found that a silica-supported tungsten oxide catalyst in combination with a co-catalyst that is magnesium oxide or a system in which sodium hydroxide is supported on γ-alumina achieve a drastically improved catalytic activity in the presence of a small amount of hydrogen in the reaction (Patent Document 4). According to this method, the reaction temperature is lowered and propylene can be produced with much higher selectivity. Moreover, provided that the reaction temperature is constant, propylene can be stably produced for a longer time of period.
However, the above method requires the presence of hydrogen, although the amount thereof is small. The hydrogen undergoes undesired reactions with ethylene or propylene, resulting in by-production of ethane or propane.
Propane that is by-produced in the above reaction reduces the purity of propylene produced. The by-production of ethane causes process problems, i.e., when unreacted ethylene is recycled into the reactor, the ethane is concentrated and accumulated in the system.
JP-B-S48-16482 (Patent Document 5) and JP-B-S57-13532 (Patent Document 6) describe that hydrogen is used in the metathesis reaction in the presence of a catalyst in which molybdenum oxide and rhenium oxide are supported on alumina. These patent documents do not address the formation of by-products by hydrogenation, and are silent on superficial velocity.    [Patent Document 1] U.S. Pat. No. 4,575,575    [Patent Document 2] U.S. Pat. No. 4,754,098    [Patent Document 3] U.S. Pat. No. 4,684,760    [Patent Document 4] WO 2006/093058    [Patent Document 5] JP-B-S48-16482    [Patent Document 6] JP-B-S57-13532    [Non-Patent Document 1] Journal of Molecular Catalysis, Vol. 28, p. 117 (1985)